Amplifier gain control circuit



Aprl 8, 1941. w. R. FERRls 2.237.420 v AggpAIEIERA-IN YCONTROL.' CIRCUITFiled June 4, 1938 A TTORNE Y.

Patented Apr. 8, 1941` OFFICE MWLIFIER GAIN CONTROL CIRCUIT Warren R.Ferris, East Orange, N. J., assigner to Radio Corporation of America, acorporation of Delaware Aminos-uen .rune 4, 1938, serial No. 211,724

6 Claims.

`ll/iy present invention relates to gain control circuits for waveamplifiers, and more particularly to a novel method of, and means for,varying'the gain of a Wave amplier without aifecting the input capacitythereof.

One of the main objects of my present invention is to eliminate shift inthe input capacity of a wave amplifier While adjusting the gain thereof;the capacity shift being eliminated by controlling the amplifierelectrode voltages in such a manner that the ratio of the transit timebetween cathode and control grid and that between control grid and thefollowing electrode has a constant value.

Another important object of this invention is to provide a method ofvarying the gain ci an amplier which includes the step of maintainingthe plate voltage to effective grid bias voltage ratio constant therebyto keep the input capacity value constant.

. Another object of my invention is to provide a method of eliminatingchange in the input capacity of an amplier as the voltage of a positiveelectrode is varied by a factor N; the method comprising varying thegrid voltage until the plate current varies by NW2.

Still other objects of my invention are to provide an amplifier of thescreen grid type wherein gain control is had by adjustment of thecontrol grid and screen grid voltages in such relative senses that thechange in gain is effooted without varying the amplier input capacity.

The novel features which I believe to be characteristic of my inventionare set forth in particularity in the appended claims; the inventionitself, however, as to both its organization and method oi operationwill best be understood by reference to the following description takenin connection with the drawing in which I havel indicateddiagrammatically a circuit organization whereby my invention may becarried into effect.

, Referring now to the drawing, there is shown a tube l of the screengrid type. The input circuit 2 thereof comprises a coil 3 and condenserli which are resonant to a desired Wave frequency. The coil 3 can becoupled to any desired source of signal Waves; for example, these wavesmay be in broadcast band of 500 to 1550 kc. The present invention isemployed with advantage in the 6o to 20G megacycle band. The tube lincludes a cathode 5, a signal control grid li, a screen grid 'l' and aplate 8. The signal output circuit 9 may feed further amplifier stages;in fact, the use of a large number of amplifier stages is desirable withthis arrangement since the amplification in each stage varies only asthe square root of the applied direct current voltage.

i plier.

Thenumeral lll denotes a source of direct current employed to energizethe various electrodes; the resistor Il is shunted across source li) toprovide a low resistance bleeder. The cathode 5 is connected to anintermediate ground point on the bleeder, While plate 8 may be connectedto the positive end thereof. Condensers l2, i3 and I4 are the usualradio frequency bypass condensers. The control grid E is connected tothe high alternating potential side of input circuit 2, While the lowpotential end of the input circuit 2 is connected to a tap l5 which isslidable usually along that portion of bleeder il which is negative withrespect to ground.

The control grid voltage is denoted by the symbol Eg. The screen grid lis connected by tap I6 to any desired point of bleeder Il which ispositive with respect to the grounded cathode. The symbol Es denotes thevoltage applied to the screen grid electrode. The control grid tocathode capacity Ci, or input capacitance, is shown by dotted lines, andis eifectively in shunt to the tuning condenser il. The control grid toscreen grid capacity Co is shown by dotted lines. The fundamentalimprovement in gain control provided by this invention resides in thefact that the frequency of input circuit 3-4 is independent of amplifiergain adjustment if the ratio of Eg to Es is kept constant; this followsfrom the fact that in the latter case Ci-l-Co is constant.

It is Well known that the input and the output capacity of an electrondischarge tube depends upon both the geometry and the potentialdistribution of the elements. The usual volume control circuits operateby applying a variable negative bias to the signal control grid of anamplifier tube, usually with the other voltages held substantiallyconstant. This results in a serious change in the input capacity of theam- For example, in the case of a 57 type tube, the change in inputcapacity is about 1 mmf., as the bias is varied from its normal value tocut-olf. This necessitates the use of large padding condensers inreceiver circuits, which reduce the possible gain to less than onehalfof its value if such padding were not necessary. Particularly in thecase of receivers equipped with automatic volume control is the changein input capacity of the controlled tube a disadvantage. In the case ofthe high transconductance tubes intended for television use the changein capacity is several micro-microfarads.

It can be demonstrated that the change in input capacity of a tube ofthe screen grid type may be expressed by the following relationship:

Cc r. n 1 ^C[1+4n 1k+1+3o+1 2)] where AC is the change in capacity from`its value at cut-off to its value at any given set of operatingvoltages; T2 is the transit time from control grid to screen grid; T1 isthe transit time from cathode to control grid; 1c is the square root ofthe ratio of the effective voltage at the screen grid to that at thecontrol grid; and Cc is the capacity between such sections of controlgrid and cathode as are in the electron stream.

It will, therefore, be seen from the above relationship that if theratio of the effective voltage at the screen grid to that at the controlgrid is kept constant, the ratio is also constant, and therefore AC ismaintained constant. This is realized, as shown in the drawing, byvarying the voltages applied to the screen grid and control grid in sucha manner as to satisfy the relations noted. It is assumed that thecontact potential of the control grid is zero, or has been balanced outby a Xed bias battery or similar means. As a matter of fact, anauxiliary bias battery can be employed in the signal grid circuit tobalance out the effect of contact potential, the initial velocity ofelectrons, thermo-couple eiTect-s, and gas ionization effects. With sucheffects balanced out, the eiective grid bias voltage of the grid 5 isthen the value E.

The present gain control arrangement is applicable to any amplifierwherein the amplier tube employs a cathode, signal grid and at least onepositive electrode adjacent the signal grid. The essential relationinvolves maintaining the positive electrode voltage to eiective gridbias voltage ratio constant thereby to maintain the input capacityconstant. This is readily shown to be the case by observing the inputcapacity values and positive electrode current under a given set ofconditions, and then increasing the positive electrode voltage by afactor N. The control grid voltage is then varied until the positiveelectrode current increases by N3/2. It can, also, be stated that theinput capacity increases with the ratio of T2 to T1, these being thetransit times noted above. In actual practice, assuming the inputcircuit 2 of the amplifier to be tuned to a given signal frequency, thegain of tube I is varied by adjusting taps I5 and I6 in such a way thatthe ratio of the transit time between cathode and control grid and thatbetween control grid and the next positive electrode, that is the plateor screen grid, shall have a constant value. If the tap I il is adjustedaway from the grounded cathode tap, then. the slider I6 is adjusted awayfrom the cathode tap. The gain is increased with such adjustment whereasmoving them towards the cathode decreases the gain. If both taps wereset positive with respect to the cathode, adjustment of both taps to theright of the cathode increases gain without change in input capacity.

After the range of gain control which is desired has been decided upon,and assuming that adjustment of the control grid tap I5 is beingdepended upon for gain control variation, the adjustment of tap I6 canbe made conjointly with adjustment of tap I 5, and in accordance withthe relationships given above, so as to maintain the tuned input circuitindependent in frequency of the amplication control.

While I have indicated and described a system for carrying my inventioninto effect, it will be apparent to one skilled in the art that myinvention is by no means limited to the particular organization shownand described, but that many modications may be made without departingfrom the scope of my invention, as set forth in the appended claims.

What I claim is:

1. A method of controlling the gain of an alternating current amplifierof the type including a tube provided with at least a cathode, a controlgrid and a positive electrode, the method including the steps of varyingthe direct current voltage of the positive electrode, varying thevoltage of the control grid in a sense 'such that the ratio of thepositive electrode voltage t0 effective grid bias voltage issubstantially constant whereby the capacity between control grid andcathode is constant.

2. A method of controlling the gain of a wave amplifier which includesvarying the direct current voltage of a positive electrode of theamplifier Which has a predominating eiect on the space current ow of thetube, and varying the direct current voltage of the control electrode ofthe tube in a sense such that the ratio of the positive electrodevoltage to effective control electrode bias voltage is substantiallyconstant whereby the input capacity of the amplier is maintainedconstant.

3. In a wave amplifier of the type including a tube provided with atleast a cathode, a wave input grid, a positive screen grid and a plate,the method which includes the steps of varying the screen grid and inputgrid direct current voltages in such relative senses as to maintain theratio of the electron transit time between the cathode and input gridand that between input grid and screen grid substantially constant.

4. In a wave amplier of the type including a tube provided with at leasta cathode, a signal grid and a positive electrode adjacent the signalgrid which has a substantial eifect on the electron flow, the method ofeliminating the input capacity shift of the amplifier during adjustmentof the gain thereof which includes adjusting the grid and positiveelectrode voltages in such relative senses that the ratio of the transittime between cathode and signal grid and that between signal grid andthe positive electrode has a constant value.

5. An amplifying system comprising a tube having a cathode, an anode, acontrol grid and a screen grid, a wave input circuit connected betweenthe control grid and cathode, a wave output circuit connected betweenthe anode and cathode, and means for controlling the gain of said tubewithout affecting the input capacitance between the control grid andcathode of the tubes, said means comprising a direct current voltagecircuit for applying control voltages to the screen grid and controlgrid, and the control grid and screen grid voltages being constantlyrelated'to prevent said input capacity shift.

6. In an amplifying system having a tube provided with at least acathode, a control grid, and a positive electrode, a means ofcontrolling the gain comprising a device arranged to vary the controlgrid and positive electrode voltages in such relative senses that anincrease in the positive electrode voltage of N-fold is made to producean increase in the positive electrode current of substantially NW2-foldthereby to prevent any change in the input capacity of the tube.

WARREN R. FERRIS.

